U.S. Pat. No. 4,513,268 to R. K. Seymour et al. describes an automated Q-line circuit breaker wherein the case is provided with a plurality of ridges and grooves for facilitating robotic assembly of the circuit breaker components. It is heretofore been customary to use a thermoset plastic material for both the circuit breaker case as well as the circuit breaker cover. The thermoset material is stable at the high operating currents which occur during limited exposure to overcurrent conditions. Since the calibration of the thermal and magnetic trip unit is critical, it is an important requirement that the components within the circuit breaker case remain in their exact locations in order to maintain calibration. The cover which is attached to the base, generally contains ribbed extensions for assisting in positioning the internal circuit breaker components in the base which must also remain intact after exposure to overcurrent and overtemperature conditions. The cover therefore has also been fabricated from a thermoset-type plastic material in order to avoid thermal distortion.
The main source of heat generated within a circuit breaker enclosure consisting of the cover and base, comprises the bimetal trip unit which is intended to become heated in proportion to the current transported through the breaker. When the breaker is subjected to two hundred percent normal operating current conditions, the temperature of the bimetal can exceed one hundred degrees centigrade. The components electrically connected with the bimetal also become heated by thermal conduction since good conductors of electricity are generally good conductors of heat. The end of the line strap is secured to one end of the bimetal and one end of the braid conductor is attached to the opposite end of the bimetal. The load strap and the braid conductor transfer heat away from the bimetal out to the load terminal and to the movable contact arm accordingly. To facilitate aligning the contact arm and guiding its motion between closed and open positions, a rib guide is formed on the internal surface of the cover. To facilitate the automation process, the braid in the automated Q-line breaker described within the referenced Patent is first installed in the case and the cradle is then positioned over the braid. It has since been discovered that the arrangement of a pair of support ridges within the case for holding the load strap and the rib guide on the cover for guiding the contact arm also function to reduce the temperature to which the cover becomes subjected upon overload test conditions. This reduction in temperature to which the cover becomes subjected has led to the use of a thermoplastic material as a substitute for the thermoset material usually employed.
The purpose of the instant invention therefore is to provide a residential-type circuit breaker case and component arrangement which allows for substitution of a thermoplastic plastic material for the cover in place of the usual thermoset plastic.